“Stands up to milk”, says the packet above a dark roast. Shelves labelled for lattes tend to hold the darker-looking leaf. Brewed at home with milk poured in, more of the tea does seem to survive. With no way to check, a second cup gets made.
What follows looks at what roast depth changes once milk is in the cup: the sellers' words, the source of astringency, the compounds behind the toasty aroma, and which way a word points.
What changes is the astringency
A soft, sweet steam rises from the cup once the milk is in. What roast depth changes most is not the aroma but the source of astringency.
In public trials, what fell as the roast deepened was the catechins. The compounds measured as roast aroma came out higher in the light roast than the dark. If a dark roast suits milk, the reason lies on the side of less astringency.
For choosing a dark roast in pursuit of aroma, current public measurements offer no support. After the answer come the sellers' words.
Roasted grain and nuts, in words
Pick up a packet in front of the shelf and a faint toasty smell escapes. In a merchant's account, the dark roast's aroma holds its own against milk.
One tea merchant recommends a dark roast because its roasted-grain, nutty aroma survives in milk. It is a seller's description, not a measured one. Even so, as a target when choosing, it is clearly put.
The sources go as far as this: that is how the seller describes it. After the words come the measured figures.
Roast deeper, less astringency
A sip leaves a faint astringency under the tongue. The deeper the roast, the lower the total of catechins, the source of that astringency.
As astringency leaves a ripening persimmon, total catechins fall as the roast deepens. In a prefectural tea station's figures, the dark roast held about a fifth of the unroasted level. Light, medium and dark were made by drum speed, not by temperature.
What falls, though, is the total. Taken type by type, some rise sharply at the light roast before falling. Roasting does not shave the astringency away evenly; it reshuffles it first, then reduces it. After astringency comes the toasty aroma.
More toastiness in the light roast
Lean over the cup and a toasty steam reaches the nose. The compounds measured as roast aroma are not greatest at the deepest roast.
In the same trial, the compounds used as a marker of toastiness are absent from unroasted tea and appear only with roasting. In first-flush tea and karibancha, the lighter the roast, the more there was. In second-flush tea, roast depth made no difference.
The sources support this direction only within that trial's conditions. Next comes another trial, one that moved the temperature.
Figure 2-2-2Roast level and the two roast-aroma pyrazines, in descending order (first-flush tea, 2013)
- Light roast
- Medium roast
- Dark roast
Not detected in aracha
Higher heat, the other way
Inside a roasting drum it is too hot to hold a hand near. In another trial that raised the temperature, the toasty compounds increased instead.
When a research institute in another prefecture roasted stems, every aroma compound singled out as important rose with the temperature. One trial moved the drum speed, the other the temperature. The conditions differ, so neither is simply wrong.
Deeper means toastier, and lighter means toastier, each hold only inside their own trial. After the trials comes which way a word on the shelf points.
The same word, facing away
Beside the tea, a bag of dark-roast coffee beans sits on the shelf. The same word, dark roast, points in opposite directions for coffee and for hojicha.
In a coffee body's account, roasting deeper moves coffee towards bitterness. Hojicha's dark roast, as just seen, moves towards less of what makes astringency. The same two characters sit on one shelf, facing away from each other.
The figures and trial conditions are gathered just below. The milk in the cup has quietly cooled a little.
The numbers in detail
- Merchant's account
- The merchant's account is FAR EAST TEA COMPANY's. It recommends a dark roast as one whose tea aroma stays distinct after milk is added.
- The trial
- The catechin trial is “Roasting Level and the Quality of Hojicha” (research period FY2013–2014), Tea Research Station, Kochi Prefectural Agricultural Technology Center. Light, medium and dark roasts were made on its continuous hojicha roaster.
- Roasting conditions
- In the 2013 table that measured catechins, the second-stage exhaust temperature was 100°C throughout. The drum speed was lowered to make the difference: 17, 12 and 8 rpm for first-flush tea.
- Catechins
- 1.9 g of leaf was steeped in 200 mL of boiling water for 5 minutes, compared per 100 mL of infusion. First-flush dark roast held about one fifth of aracha's level; karibancha fell below one tenth.
- By type
- Gallocatechin gallate rose more than tenfold at the light roast, then fell through the medium and dark.
- Pyrazines
- The two pyrazines used as a roast-aroma index were not detected in aracha. They were highest at the lighter roasts in first-flush tea and karibancha, with no difference by roast level in second-flush tea.
- Temperature trial
- The temperature trial is by the Industrial Research Institute of Ishikawa and others. Stems of Kagoshima first-flush tea were roasted at drum inner-surface temperatures of 180–220°C. All eight pyrazines identified as key aroma compounds rose at higher temperatures.
- Coffee
- The account of coffee roasting and bitterness is from the All Japan Coffee Association.